fileno(3) differs from the other functions in various ways.
For example, it is governed by different standards,
and can set 'errno'. Conversely, the other functions
are about examining the status of a stream, while
fileno(3) simply obtains the underlying file descriptor.
Furthermore, splitting this function out allows
for some cleaner upcoming changes in ferror(3).
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>